Question

What is the electron configuration of the $A{s^{3 - }}$ ion?

Answer 1

Hint : We can find the configuration of any given element if we know its atomic number or mass number. The configuration tells us which electron takes which orbital and shells, using the laws for writing the configuration. Here the element given to us is Arsenic and since they have specified that we need to give configuration for the $A{s^{3 - }}$ ion, it means that three electrons have been added to the orbit of this ion.

Complete Step By Step Answer:
Keep in mind that the electrons can only be added to the lowest energy orbital by the Aufbau principle, then every orbital will accommodate only at most a pair of electrons having spins that are opposite by Pauli’s exclusion rule and the subshells can only be filled one by one, not directly in pairs by Hund’s rule.
Let us understand that the atomic number of this element Arsenic $(As)$ is $33$ . But we already know that atomic number will be exactly the same as proton count also, and electron count for the neutral atom of $As$ shall be $33$ .
For the neutral atom the configuration is: $[Ar]3{d^{10}}4{s^2}4{p^3}$ .
Then to give configuration for the $A{s^{3 - }}$ ion, it means that three electrons have been added to the orbit of this ion. So we just need to add $3$ more electrons to the $p$ subshell, since its capacity is $6$ .
$\therefore$ Electronic configuration of $A{s^{3 - }} \Rightarrow [Ar]3{d^{10}}4{s^2}4{p^6}$ .

Note :
Keep in mind that always electrons first enter the $4s$ (lower energy) orbital before moving on to the 3d orbitals. As electrons occupy the $3d$ orbitals, they repel the $4s$ orbitals to a slightly higher energy level, which must be written down correctly when writing down the electronic structure for atoms. There are exceptions like the configurations of $Cr$ and $Cu$ , since $3d$ subshells that are half filled or completely filled are much more stable.